So-called "cellular" nets have fixed stations spread out over the ground, with each fixed station being responsible for establishing communication with mobiles in a region, known as a "cell", surrounding each fixed station.
Conventional radio nets use channels which are defined by the values for their center frequencies; information is transmitted by narrow band analog modulation, frequency modulation, amplitude modulation, or single side band. In nets having a large number of mobile subscribers, the frequency channels are not allocated to specific groups of links, but are held in common to be allocated to a calling mobile as a function of traffic. In such a system, channel management is important, since the bulk of the protection against interference between different calls is obtained by allocating different frequencies to links which are geographically close to one another.
The increasing numbers of customers applying to be connected as mobile radiotelephone subscribers require new radio net structures that enable greater density in the use of the available spectrum, or more precisely that provide greater spectrum efficiency which, by reference to telephony, can be measured in erlangs/hertz/km.sup.2.
A first method of increasing spectrum efficiency consists in reducing the range between the mobiles and the nearest fixed station, but there is a limit to the improvement which can be obtained in this manner. Present systems are moving in this direction.
Preferred embodiments of the present invention provide a radio communications system which is capable of obtaining greater spectrum efficiency, which is highly flexible in installation and use, and which reduces infrastructure costs.
To do this, radio communications systems in accordance with the invention use a system of dispersing power by frequency spreading which enables higher spectrum efficiency to be obtained in spite of the apparant chaos caused by superposing signals from different links in the same space-time-frequency continuum without constraint. The power dispersion technique used in radio communications systems in accordance with the present invention is frequency hopping, with each mobile station having its own frequency-hopping pattern known to the fixed stations. When there are several fixed or master stations they may readily be synchronized by simple means.